1. Field of the Invention
The present invention relates to a speed control apparatus for an electric motor. More specifically, the present invention relates to a speed control apparatus for a sewing machine driven by an electric motor energized by an alternating current as phase controlled by a voltage controlled switching device.
2. Description of the Prior Art
Heretofore, a sewing machine adapted to be driven by a universal motor has been put into practical use. For the purpose of speed control of such motor, an alternating current, phase controlled by means of a voltage controlled bidirectional switching device, such as a triac, is applied to a universal motor for energization. On the other hand, a situation is encountered in such an electrically driven sewing machine where an overload is caused to the motor at the time when the needle penetrates into thick layers of cloth to be sewn, thereby decreasing the speed and rotational torque of the motor. In order to eliminate such a situation, it has been proposed that in phase controlling the alternating current by means of a voltage controlled bidirectional switching device, such as a triac, an induced voltage or a counter electromotive force of the motor is detected to be utilized additionally to control the conduction angle of the voltage controlled bidirectional switching device. More specifically, if and when a motor is subjected to an overload, the rotational speed decreases and, accordingly, the induced voltage across the motor diminishes. Therefore, the decrease of the induced voltage across the motor is detected and is utilized to increase the conduction angle of the voltage controlled bidirectional switching device, thereby to increase the rotational torque and the speed of the motor. However, a conventional circuit configuration for detecting the induced voltage across the motor for controlling the conduction angle of the triac is rather complicated, which makes the apparatus expensive.
As well known, for the purpose of speed control of an electrically driven sewing machine, a power supply is applied to a foot controller and a driving voltage, as controlled by the foot controller, is applied to an electric motor. A typical speed control of an electric motor comprises a so-called resistor control, which may comprise a carbon pile, a winding resistor and the like. According to such a resistor control of an electric motor, only two lead wires are required for connection between the power source and the foot controller and between the foot controller and an electric motor circuit. However, where a voltage controlled bidirectional switching device, such as a triac, is utilized for the purpose of speed control of an electric motor, a further additional lead wire is required in order to feedback the induced voltage to the voltage controlled switching device in the foot controller as a control voltage. Thus, in a speed control of an electric motor by means of a voltage controlled bidirectional switching device for phase control, lead wires for power supply cannot be shared for supply of a control signal to a voltage controlled bidirectional switching device.